Abstract Cr(VI) is a human carcinogen of significant public health concern, and a substantial exposure in a number of occupational settings. Cr(VI) induces mutations, changes in gene copy number, and exposure has been associated with humans cancers in exposed populations and animal models. Our novel preliminary data demonstrate that Cr(VI) exposure causes amplification in ribosomal DNA (rDNA) copy number and changes in the nucleolus (the crudely understood nuclear organelle that is the site of ribosomal RNA (rRNA) transcription, and integration of myriad cellular functions). A crucial element of nucleolar function is rDNA copy number (rDNA CN). rDNA CN modulates (i) epigenetic states across the genome, (ii) DNA damage responses, (iii) cell cycle progression, (iv) chromosome segregation, and (v) global genetic stability. Furthermore, disruption of rDNA arrays, ribosome biogenesis, and the nucleolus are central to carcinogenesis. Our central medical hypothesis is that Cr-induced changes in rDNA CN are responsible for Cr-induced carcinogenesis. Our central basic hypothesis is that rDNA arrays are not fixed, but rather a genetically dynamic component of the nuclear genome with copy number that is modulated by Cr exposure. Our proposal examines rDNA changes upon Cr(VI) exposure to reveal a novel pathway of Cr toxicity with medical and basic relevance. Key elements are a careful investigation of the toxicology of Cr-induced-rDNA-amplification (Cr-i-rDNA-a), hypotheses-driven functional genomic analysis the rDNA and the nucleolus upon Cr(VI) exposure, and extensive genetic analyses of Cr-i-rDNA-a using a powerful model organism. Our first aim will investigate the toxicology of Chromium- induced-rDNA-amplification (Cr-i-rDNA-a) in a human lung epithelial cell model. We will determine dose- responses of Cr-i-rDNA-a, map amplification boundaries in Cr-i-rDNA-a, examine temporal profiles and recovery from Cr(VI) exposure, and examine whether Chromium-induced CN changes are responsible for Cr- induced carcinogenesis. Our second aim investigates the functional genomics of Cr-induced nucleolar stress and Cr induced transformation in a human lung epithelial cell model. Examining genome-wide responses to Cr exposure is critical to understand how Cr induces rDNA amplification, nuclelar stress, and carcinogenesis. Our third aim addresses the genetic determinants of Chromium-rDNA interactions. We will examine Cr-i-rDNA-a in specific cells, quantify the extent of copy number change, isolate the affected tissues, and use high-throughput techniques to characterize the changes. Our efforts will shed light on Cr-rDNA interactions, with research that is directly relevant to the human health mission of the NIH. The manifold effects of rDNA CN indicate that perturbing this central regulator with Cr will have profound consequences to cellular function. We anticipate that determinants of complex human diseases with strong environmental components such as cancer will ultimately be traced to environmentally triggered variation in rDNA segments of the genome.